1. Field of Invention
The present invention relates to the fields of petroleum reservoir development research and computational fluid dynamics, and provides a numerical simulation method for characterizing fluid channeling along large-aperture fractures of reservoirs. In the present invention, the “geometric similarity” of large-aperture fractures is achieved by grid division of reservoir to characterize the actual distribution of large-aperture fractures; and the “hydraulic similarity” of large-aperture fractures is achieved by adjusting the permeabilities of the grid faces crossed by large-aperture fractures, and the transmissibilities of the adjusted grid faces must be equal to the actual transmissibilities of large-aperture fractures; furthermore, the traditional numerical simulation software is improved to be an unstructured grid simulator. As a result, the rapid fluid channeling along large-aperture fractures of reservoirs can be reflected very well with the invented numerical simulation method.
2. Description of Related Arts
During oil field development process, it has been found that water breakthrough occurs late in the production wells which are close to the water injection wells, while occurs soon in the production wells far away from the water injection wells. Such phenomenon is often caused by water channeling along large-aperture fractures. How to represent the rapid fluid channeling in large-aperture fractures has been an important technical problem and also a hot research topic in the field of numerical reservoir simulation.
In recent years, several commercial numerical simulation softwares have developed an equivalent simulation method for characterizing rapid fluid channeling along large-aperture fractures; however, our tests found that this processing method is not so valid. The success and effectiveness of the examples for the softwares is due to the use of “non-neighbor connection” (NNC) technique. In the NNC method, the grids which have free water are directly connected to the perforated grids of the production wells, consequently two grids spatially far away from each other are made artificially to be “directly adjacent”. Obviously, this method can not satisfy the criteria of the geometric similarity and hydraulic similarity in reservoir simulation.
In last several years, the discrete fracture network simulation technique has been in the ascendant and become popular in the numerical simulation of fractured reservoirs. Literature shows that the discrete fracture network simulation technique includes two processing methods. In one of the methods, the discrete fracture network geological model is first built according to the relevant parameters of individual discrete fracture of the intersecting fracture network, including fracture aperture, fracture extension direction, fracture length and so on; then the built discrete fracture network geological model provides input data for the dual porosity simulation model by relevant geological modeling software. The practical applications show that this processing method can not effectively characterize the rapid fluid channeling along large-aperture fractures. The reason is that a series of zigzag grids crossed by large-aperture fractures are used to describe the distribution of the fractures, although the hydraulic similarity of the fractures can be achieved by the equivalent treatment of the transmissibilities of large-aperture fractures, it is difficult to represent the actual geometric distribution characteristics of large aperture fractures (it is particularly true when the dimensions of the simulation grids are great). Thus this processing method can not meet the geometric similarity criterion.
The main idea of the other discrete fracture network simulation method is described as follows. Firstly, the flow equations of discrete fracture network system are given, which comprise the mass balance equations of all discrete fracture network nodes and all fracture segments, here a discrete fracture network node is a point where two or more fractures intersect or a fracture ends, and a fracture segment is the portion of a fracture which lies between two nodes. Secondly, the flow equations of matrix system must be given. Then the flow equations of discrete fracture network system are solved by analytic method based on Laplace transform. Lastly, on the basis of the computed fracture network variables, the matrix system flow equations are solved to obtain relevant variable values of the matrix system. This processing method can simultaneously meet the geometric similarity and hydraulic similarity simulation criteria. Theoretically, it can effectively represent the rapid fluid channeling along large-aperture fractures. However, the corresponding application examples of this processing method have not been reported in the literature. Furthermore, this processing method can not reflect the variation of fluid pressure and saturation along the same fracture segment, and the errors of the simulation results increase with the length of a fracture segment. Moreover, it is difficult to implement since the complicated Laplace transform and inverse Laplace transform are involved during the solving process.
Aiming at the solution to the above simulation problems, the present invention advances a new discrete fracture network processing method for numerical reservoir simulation, which is relatively simple in principle and easy to implement. Firstly, during the numerical simulation process of a reservoir with large-aperture fractures, each large-aperture fracture is treated with the “geometric similarity criterion”, adopting the unstructured grid division to ensure that the so-called fracture grids, which are crossed by large-aperture fractures, can reflect the actual distribution characteristics of every large fracture (including the length, the extension direction and aperture), thus the geometric similarity of large-aperture fractures can be achieved. Secondly, the transmissibility of the grid face crossed by each large-aperture fracture must be equal to the actual transmissibility of corresponding large-aperture fracture, hence the hydraulic similarity of the large-aperture fracture can be obtained. Furthermore, the conventional dual porosity simulation software is improved to be an unstructured grid simulator. The application examples have demonstrated that the simulation method of the present invention can effectively characterize the rapid fluid channeling along large-aperture fractures.